1. Field of the Invention
The present invention relates to a discharge electrode for a wire bonding apparatus and more particularly to a discharge electrode for a wire bonding apparatus that has an insulating film on the surface of the electrode.
2. Prior Art
In a wire bonding apparatus, for instance, bonding pads formed on a die such as an LSI (large-scale integrated circuit), etc. and bonding leads of a circuit board are connected by, for example, a fine gold wire. At initial bonding in such a gold wire to a bonding pad, the tip end of the gold wire that passes through a capillary is formed into a ball shape. The ball-shaped entity that is formed on the tip end of this gold wire is called an initial ball.
FIG. 4 shows how the initial ball is formed in a conventional technique.
The tip end of a discharge electrode 10 is bent, and this discharge electrode 10 is set in a position that faces the tip end of a wire 14 that passes through a capillary 12. When a high voltage in which the discharge electrode side is placed at a negative polarity is applied across the discharge electrode 10 and wire 14 by a high voltage generator 16, a space discharge occurs between the facing tip end of the discharge electrode 10 and the wire 14, so that the tip end of the wire 14 is melted, thus forming an initial ball. In this way, the initial ball is formed by a discharge 18 that occurs between the discharge electrode 10 and the tip end of the wire 14.
In order to obtain a stable discharge and form a stabilized initial ball shape by limiting the location of the discharge on the discharge electrode, the surfaces of the discharge electrode other than the discharge surface are typically covered by an insulating film. Various types of films such as tetrafluoroethylene films, alumina coating films, fluororesin coating films, silicon oxide or alumina oxide are used as insulating films as disclosed in, for instance, Japanese Patent Application Laid-Open (Kokai) Nos. S61-279140 and H4-263442. A stable discharge is also obtained by bending the discharge electrode as shown in FIG. 4 (and as disclosed in Japanese Patent Application Laid-Open (Kokai) No H4-263442), thus making a clear difference in the discharge distance.
The reason that various types of films are used as coating films for discharge electrodes as described above is that the insulating films have respective advantages and disadvantages in terms of heat resistance, insulating properties and coverage of indentations and projections in the external shape of the discharge electrode, etc. For example, in the case of resin films, there are difficulties in terms of durability, depending on the material, gases may be generated by the high heat during discharge, thus clouding the optical system of the wire bonding apparatus or causing a deterioration in the elements that make up the system, such as solidification of the gases and adhesion to the bonding head, etc.
Furthermore, alumina films have stable physical properties and chemical properties, and they are considered as insulating films that have good compatibility with discharge electrodes that generate a high heat during discharge. However, while such films have a sufficient electrical resistance in terms of contact resistance, these films have a low resistance with respect to discharges because of their porous structure. In other words, such films suffer from the following drawback: a discharge occurs at a low voltage from thin portions of the film that has a porous structure, and when a continuous discharge occurs, the surrounding film is destroyed.
A number of techniques for sealing the pores of porous structures (centering on film formation techniques) are known. However, all of these techniques concentrate on characteristic evaluation as insulating films, with no mention of resistance to discharges, etc. For example, in one of the reports of the Aichi Industrial Technology Institute written by Kataoka et al. under the title of xe2x80x9cInsulating Property of Plasma Sprayed Alumina Coatings Sealed by Laser PVD Filmxe2x80x9d and posted in 1994 on the Internet web page of Aichi Industrial Technology Institute, sprayed coating films were covered by a PVD (physical vapor deposition) film, and the insulating resistance was evaluated; however, no evaluation regarding resistance to discharges was indicated in this report.
Conventional techniques thus have problems involving generation of undesirable gases in the coating film of the discharge electrode by the discharge or a low resistance to discharges, etc. Accordingly, the ability to withstand a discharge is insufficient, a stable discharge cannot be ensured, and a stabilized shape for the initial ball likewise cannot be ensured.
In the methods that limit the location of discharge by way of bending the tip end of the discharge electrode in order to cover for an insufficient ability to withstand a discharge, it is necessary to raise the capillary by an excess amount that corresponds to the amount of bending in order to form an initial ball by a discharge. As a result, the wire bonding treatment speed drops, and there are restrictions on the range of mobility of the area around the bonding head, including the capillary.
Accordingly, the present invention solves the above-described problems encountered in the prior art.
It is an object of the present invention to provide a discharge electrode for a wire bonding apparatus that can ensure a stable discharge.
It is another object of the present invention to provide a discharge electrode for a wire bonding apparatus that can improve the resistance with respect to discharges.
The above object is accomplished by a unique structure of the present invention for a discharge electrode used in a wire bonding apparatus that applies a high voltage across an electrode and a tip end of a wire bonding wire so that a discharge is generated between the electrode and the wire; and in the present invention, the discharge electrode is comprised of:
a conductive electrode core material; and
an insulating film that has a porous structure and is formed on the surface of the electrode core material, the insulating film being formed with an insulating layer which is obtained by having the porous structure subjected to a pore sealing and which covers the electrode core, and the electrode having, on a portion of its discharge position that faces the tip end of the wire, an exposed surface where the electrode core material is exposed.
In the structure above, the electrode core material is covered by an insulating film in which the pores of its porous structure are sealed, except in the area of the discharge position that faces the tip end of the wire. The insulating film that has a porous structure has a low resistance with respect to discharges; however, since in the present invention the porous structure is subjected to a pore sealing treatment, the resistance with respect to discharges improves. Accordingly, a discharge is generated only on the exposed surface of the discharge position, so that a stable discharge is ensured.
In the above discharge electrode, it is preferable that the insulating film that has the porous structure is a porous alumina film that is formed by an anodic oxidation method or a plasma flame coating method. In addition, the insulating layer formed by the pore sealing treatment is preferably an insulating layer that is formed by depositing an inorganic material film on the surface of the porous alumina film by laser PVD. Since the pores of the alumina film that has a porous structure are sealed by the inorganic material film, the alumina film, which has stable physical characteristics and chemical characteristics and which has good compatibility with a discharge electrode that generates a high heat during discharge, is used in the present invention as an insulating film of the discharge electrode.
Furthermore, the insulating layer formed by the pore sealing treatment is an insulating layer formed by impregnating the porous alumina film with a resin. The pores of the alumina film that has a porous structure are thus sealed by resins in the present invention, and a material that shows little out-gassing is preferably employed as the resin that embeds the porous structure.